1. Field of the Invention
The present invention relates to an exposure apparatus which illuminates an original with light from a light source, and projects the pattern of the original onto a substrate by a projection optical system, thereby exposing the substrate.
2. Description of the Related Art
Japanese Patent Publication No. 61-34252 discloses an exposure apparatus having an arrangement which detects, by a photoelectric detector, the intensity of light from a recording target or light applied to a recording target to obtain a series of output pulses having a frequency corresponding to the detected value, counts the number of output pulses, and closes a shutter when the counted number of pulses has reached a predetermined number of pulses. The shutter includes one rotating disk on which light-shielding and light-transmitting portions are formed alternately, and is rotationally driven to control the states, in which the illumination light is shielded and transmitted.
In exposure control with a small exposure dose, an arrangement as disclosed in Japanese Patent Publication No. 61-34252 often fails to close the shutter in time. To combat this problem, the following methods can be adopted. One method closes the shutter without counting the number of pulses. Another method counts the number of pulses in the above-described way while the light intensity is low, and closes the shutter when the counted number of pulses has reached a predetermined number of pulses.
However, the method of closing the shutter without counting the number of pulses must not be adopted if the required exposure dose accuracy is relatively high although it is advantageous to improve the throughput if the required exposure dose accuracy is relatively low. Also, the method of decreasing the light intensity is disadvantageous in respect of the throughput if the required exposure dose accuracy is relatively low although it is useful if the required exposure dose accuracy is relatively high.